Blockade of the enzyme PDE9 prevents degradation of the molecule cyclic GMP, which has been shown to protect against heart failure. The finding indicates that PDE9 inhibition might be a drug target for treating this condition. See Letter p.472
Chronic heart failure, a disease in which the heart cannot pump sufficient blood to meet the needs of the body, is a leading cause of death worldwide. Persistent high blood pressure is an important risk factor for heart failure, because it increases the heart's workload, which in turn increases the size and strength of heart-muscle cells called cardiomyocytes — a condition known as cardiac hypertrophy. In the long term, this can cause permanent molecular and structural changes in cardiomyocytes, impairing cardiac contraction and relaxation. To counteract this risk, the heart releases natriuretic peptide hormones that lower blood pressure and exert direct cardioprotective effects, but natriuretic-peptide signalling is attenuated in heart failure. In this issue (page 472), Lee et al.1 demonstrate that restoration of this signalling may be a useful strategy for treating heart failure.
Understanding the molecular mechanisms that regulate cardiac hypertrophy and heart failure is a major focus of research in cardiovascular medicine. Numerous studies2 have shown that an intracellular molecule called cyclic GMP (cGMP) provides protection from high blood pressure and cardiac disease, in part by activating the enzyme cGMP-dependent protein kinase type I (PKGI), which modulates the activity of many target proteins. cGMP has a crucial role in regulating fundamental cellular processes throughout the body2, including acute processes such as cell contraction, migration and secretion, and chronic processes such as gene expression and cell growth.
In cardiomyocytes, as in most cardiovascular cell types, cGMP production is promoted by four hormones, acting through three receptor proteins found in different subcellular locations2. First, the gaseous hormone nitric oxide (NO) stimulates a cytoplasmic, soluble guanylyl cyclase (sGC) receptor. Second, atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) hormones activate a guanylyl cyclase type A (GC-A) receptor that spans the cardiomyocyte plasma membrane. Finally, C-type natriuretic peptide (CNP) hormone binds to a membrane-spanning guanylyl cyclase type B (GC-B) receptor. Both NO and CNP are secreted from endothelial cells that line the blood vessels, whereas ANP and BNP are produced by cardiomyocytes themselves (Fig. 1). Opposing the action of these hormones, the phosphodiesterase enzymes PDE5 and PDE9 degrade cGMP by hydrolysis and so regulate the duration, amplitude and spatial distribution of cGMP signalling within cardiomyocytes3. Such spatial restrictions mean that cGMP-activated PKGI can regulate a range of cardiomyocyte functions4.
Both NO and natriuretic peptides, acting through their respective receptors to promote cGMP formation in cells of the kidney and blood vessels, lower blood pressure and thereby reduce cardiac work5. Studies4,6 suggest that cGMP also has a protective function in the heart, accelerating relaxation, decreasing the stiffness of cardiomyocytes and moderating adverse cardiac remodelling. The accumulating evidence of cGMP's role in the heart, and of a protective role in other organs and tissues, such as the brain, lung and vasculature, led to an interest in drugs that might enhance cGMP signalling, and much attention has focused on PDE5 inhibition. Since 1998, three PDE5 inhibitors have been used to treat erectile dysfunction and pulmonary hypertension. Preclinical studies with one such inhibitor, sildenafil, showed dramatic cardiac anti-remodelling benefits in animals, and these were partly confirmed in small human studies7,8. But a large clinical trial in patients with heart failure reported disappointing results9.
Lee et al.1 investigated the cellular distribution of PDE5 and PDE9 in cardiomyocytes, and showed that PDE5 is found at contractile filaments called myofilaments, where it degrades cGMP produced through the NO–sGC pathway. By contrast, they showed that PDE9 is located near 'T-tubular' invaginations of the plasma membrane and mainly regulates cGMP produced by the ANP–GC-A pathway (Fig. 1). The authors also found that PDE9 levels and activities, like those of PDE5, rise in hypertrophic cardiomyocytes, both in mice and in patients with heart failure. The proteins spill out into abnormal intracellular compartments, increasing cGMP degradation.
Oxidative stress provokes a dysfunctional uncoupling of the NO-producing enzyme in patients with heart failure, leading to the production of noxious oxygen radicals instead of NO. Lee and colleagues reasoned that this effect, which attenuates cGMP production through the NO–sGC pathway, could explain the limited clinical effectiveness of PDE5 inhibitors. But natriuretic-peptide-driven cGMP synthesis is also compromised in the cardiomyocytes of patients with heart failure — GC-A receptors become desensitized, and the cells secrete large amounts of precursors to ANP and BNP, which are not properly processed and so are poorly active2. The authors therefore propose that inhibiting PDE9, to enhance the pool of cGMP derived from the ANP–GC-A pathway, is an attractive alternative to inhibiting PDE5.
The authors explored this possibility by using a mouse model of cardiac pressure overload. Mice do not normally develop high blood pressure or cardiac hypertrophy, so the researchers surgically constricted the aorta, which carries blood away from the heart, thereby artificially enhancing cardiac 'afterload'. Within a few weeks of surgery, the hearts of mice with aortic constriction were approximately 60% larger than those of control animals. This was accompanied by the formation of fibrous tissue and by reduced heart contraction and relaxation, all features of early-stage heart failure. Remarkably, genetic or pharmacological inhibition of PDE9 (using a compound called PF-04447943) prevented and even reversed existing heart failure in these mice.
The authors' suggestion that modulating the pool of ANP- or BNP-derived cGMP can benefit patients with heart failure is supported by research showing that inhibiting neprilysin — a peptidase enzyme that degrades ANP, BNP and other hormones — enhanced endogenous ANP and BNP levels, and reduced the risks of hospitalization and death in patients with heart failure10.
Other strategies for treating heart failure are also worthy of consideration. For instance, one approach under investigation is the use of synthetic 'designer' natriuretic peptides11. The CNP–cGMP pathway and corin, an enzyme that activates cardiac ANP, may also represent targets for heart-protecting therapies.
PF-04447943 is being tested in clinical trials for neurocognitive diseases (see go.nature/a8qtw9), and seems to be well tolerated in humans. Lee and colleagues' exciting observations in mice, when considered together with the fact that older patients with heart failure frequently exhibit cognitive impairment12, support the exploration of PDE9 as a target for treating heart failure.
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Journal of Molecular and Cellular Cardiology (2016)